1. Field of the Invention
The present invention relates to electric motors, and more specifically is concerned with a piezoelectric motor.
The piezoelectric motor of the invention is intended for wide application as a micropower motor of actuating means in automatic systems, or as a motor for tape transport mechanisms in tape recorders, photographic equipment, servosystems, and various domestic appliances.
2. Prior Art
As present there are known piezoelectric motors which convert electric energy into mechanical energy in the form of angular motion or continuous rotation of the rotor thereof.
U.S. Pat. No. 4,019,073 discloses a piezoelectric motor comprising a stator which is rigidly mounted with respect to the motor support, and a rotor which is a movable member of the motor and is rotatably mounted, for instance, in bearings on the stator.
Unlike electric motors utilizing electromagnetic interaction between the stator and rotor, a piezoelectric motor operates on frictional interaction between the stator and rotor, that is at the expense of frictional forces. Therefore, when de-energized the rotor is kept at rest by a certain frictional torque and to make it rotate it is necessary to apply a moment of force which overcomes a frictional force. The torque in the stator or rotor of a piezoelectric motor is developed by a piezoelectric oscillator (also known as a piezoelectric vibrator) which is an electromechanical resonator incorporating a piezoelectrically active member, that is a piezoelectric element. The piezoelectric element is made from mono- or polycrystalline piezoelectric material which is electrically polarized in one direction. It also includes at least two electrodes made, for instance, in the form of thin metal coating, to which electrodes are generally connected metal leads adapted to connect the piezoelectric element to an alternating voltage source, with the frequency of the power source being normally selected equal or close to the resonant frequency of the oscillator.
When the piezoelement is connected to the above-type power source elastic vibrations are produced in it at the expence of the piezoelectric effect, which vibrations cause the whole oscillator to vibrate, said oscillator being acoustically insulated within the stator.
The rotor and stator in the prior art piezoelectric motor are urged against each other at the location of the working surface of the piezoelectric oscillator to provide their frictional interaction along the surface formed by rotation of at least one length of a straight line. The term "working surface" of the piezoelectric oscillator is used to mean that surface of said oscillator which is subject to wear when in use. On said frictional interaction surface there takes place conversion of the oscillator mechanical oscillations into a unidirectional tangentional force which varies in magnitude and which when applied to said surface causes the torque to develop. In other words the frictional contact zone functions as a rectifier to convert alternating mechanical stress into pulsating one.
The amount of stress in the oscillator and oscillation frequency thereof determine the torque and r.p.m. of the rotor in a piezoelectric motor. The stress and mechanical oscillations in the oscillator are related to each other through a coefficient of elasticity of the material used. Therefore, the torque and speed of rotation of the rotor in a piezoelectric motor have a linear relationship therebetween, that is the piezoelectric motor has a drooping load characteristic. This, however, cannot be considered as a disadvantage of a piezoelectric motor, since it is a drooping characteristic which is frequently required. In some cases, however, a flat characteristic is required, which characteristic is generally obtained with the aid of electronic circuits adapted to stabilize the r.p.m. of a piezoelectric motor. Furthermore, motors used in tape recorders and electric record players must have step variation of the rotational speed which is preferably effected without lowering its torque, since otherwise a detonation coefficient will markedly increase. It is to be noted in this connection that the prior art piezoelectric motor construction does not practically allow a stepwise variation of the rotational speed without changing the rotational moment.
Another disadvantage of the prior art piezoelectric motor is that its construction does not provide a wide range of the r.p.m. variation, as with the speed decrease the rotor motion becomes more and more irregular, which is also caused by the torque decrease, in which case the torque becomes commensurable with the load torque variations.
In addition, the problem of the rotational speed reverse has not been completely solved in the prior art piezoelectric motor. As a matter of fact non-reversal motors have high load torques, high efficiency, and a long service length. They feature a low noise operation, and are supplied from simple electric supply circuits. The reverse in the prior art piezoelectric motor is effected by exciting mechanical oscillations of two types, that is longitudinal and transverse ones, in which case the piezoelectric element must have two layers which complicates to a great extent its electrode topology and polarization. Such construction also affects Cos .phi. of the piezoelectric element, decreases piezoelectrically active zone, and hence causes higher voltage supply. The presence of several layers bonded with each other affects the strength of the piezoelement, and flexural vibration occurring in monomorphic piezoelectric elements deteriorates load matching therein and brings down its efficiency as a result. At present the efficiency of reversible piezoelectric motors does not exceed 10% which does not allow these motors to meet many technical requirements, whereas the efficiency of non-reversible piezoelectric motors may exceed 80%. The service life of the piezoelectric motors having rotors made from superhard materials does not exceed 100 hours, while the service life of the non-reversible motors is in excess of 1,000 hours.
A further disadvantage of the prior art piezoelectric motor is that is has one shaft. In some cases, however, there is a need that the motor have two or more shafts with different rotational speeds. For instance, the use of a two-shaft piezoelectric motor in a tape transport mechanism of tape recorders may considerably simplify kinematics of such mechanisms.